A ground-based missile interceptor is lowered into its silo during an emplacement at the Missile Defense Complex at Fort Greely, Alaska, in 2007. (Sgt. Jack W. Carlson III/US Army)

WASHINGTON — The Missile Defense Agency is looking at five potential locations to house a controversial third domestic ground-based interceptor site, which would guard the continental United States against ballistic missile attack.

While a site hasn’t been chosen, whittling the potential locations down to a few sites will allow to Pentagon to begin environmental and other assessments if Congress provides the money to go ahead with the build.

In a statement on Sept. 12, MDA director Navy Vice Adm. James Syring said that “while the administration has not made a decision to build another missile defense facility in the U.S. for homeland defense, if a decision were to be made in the future to construct a new site, completing the required site study and environmental impact statement would shorten the timeline required to build such a site.”

All of the sites are already on federal land:

■ Fort Drum, N.Y.

■ Camp Ethan Allen Training Site, Vt.

■ Naval Air Station Portsmouth SERE Training Area, Maine

■ Camp Ravenna Joint Training Center, Ohio

■ Fort Custer Training Center, Mich.

Despite the fact that his state is being considered for the site, Sen. Patrick Leahy has said that he considers the program to be a waste of money, and he opposes placing it in his state.

John Isaacs, director of the Council for a Livable World, said in a statement that “the United States should not rush to deploy a missile defense site on the East Coast until a need for such a site is identified and the interceptors to be deployed at the site prove effective and suitable in operationally realistic tests.” The group is a non-partisan organization focused on nuclear weapons proliferation.

The U.S. already operates GBI sites at Fort Greely, Alaska, and Vandenberg Air Force Base, Calif., with 30 GBIs on line, and another 14 to be added by 2017.

The issue of an additional GBI site on the East Cost sparked controversy on Capitol Hill this summer, as Senate Democrats pushed back against congressional Republicans, who included money in their 2013 defense budget markup for the site.

It was further complicated by the MDA launching yet another failed test of its existing interceptors, marking a third failed intercept test in the past five years.

In a written reply to Sen. Carl Levin this past June, Syring, along with Lt. Gen. Richard Formica, commander of the Joint Functional Component Command for Integrated Missile Defense, admitted that there is “no validated military requirement” for a proposed East Coast missile defense site.

The letter came in response to one Levin sent to the two officers asking if there was an urgent need to begin work on a third site. In its 2013 budget markup, the Republican-controlled House Armed Services Committee voted to set aside $250 million for the construction of a missile defense system on the East Coast, making its second attempt to get the site into the budget after having a similar proposal shot down by the Senate Armed Services Committee last year.

The proposal from the House comes at a time of increased worry about North Korean, Chinese, and Iranian ballistic missile threats against the mainland United States and its allies, even though many analysts say that neither the North Koreans nor the Iranians are close to having the ability to hit the United States.

Nevertheless, in March Secretary of Defense Chuck Hagel announced that he was earmarking about $1 billion to fund the emplacement of 14 additional missile interceptors in Alaska to guard against a missile attack from North Korea. The additional interceptors would bolster the 26 already deployed in Alaska and four in California, and give the United States 44 interceptor sites in all.

But in July, Syring said that the government wants even more. “The 44 [is for] what we see with North Korea today,” he said, adding that there is the real potential “to go beyond 44 as we start to evaluate the threat from Iran and from other nations.”

The Congressional Budget Office has estimated that expanding the ground-based midcourse defense system to the East Coast would cost about approximately $3.5 billion over the next five years.